The current microneedle array may be made of silicon (Si substrate), metal or polymer. The manufacturing methods of Si substrate microneedle array can further be categorized as using wet etching or dry etching. The manufacturing process of metal microneedle array can further be categorized as using electroplating or deposition. The manufacturing process of polymer microneedle array can be further categorized as using molding or photolithography.
Among the methods of microneedle array, the most widely adopted is using Si substrate to fabricate the hollow needles or mold. However, the fabrication process of using Si substrate is more complicated, as disclosed in WO0217985, and requires many steps of wet/dry etching and thin film deposition. As it takes a longer time to fabricate, the yield rate is low and the cost is high. U.S. Pat. No. 6,334,856 disclosed a method of fabricating a microneedle array having flat needle tips and tapered tubes, as shown in FIG. 1. This type of design limits the width of the flow channel and the flexibility of the needle. To fabricate the needle higher than 100 um, the needle density must be restricted in compromise for an appropriate size of aperture and strength of needle structure. The restriction of low needle density further causes the problem of insufficient sampling. In addition, the Si substrate microneedles are brittle and break easily.
The tip of the hollow microneedle in most prior arts is designed as flat, except the design disclosed in WO0217985 (see FIG. 2), which is a slant. This is because a slant tip is easier to penetrate the human skin for micro-sampling than the flat tip, as the human skin is flexible.
Kim et al. disclosed a method for fabricating metal microneedle array in Journal of Micromechanics and Micro engineering in 2004. They spread two layers of SU-8 on a glass substrate and used a back exposure to seperately bake the two layers of SU-8. They also used reactive ion etching to obtain an SU-8 pillar array structure, and then used sputtering, electroplating, planarization and polishing to fabricate a tapered metal hollow microneedle array, as shown in FIG. 3. However, the method requires multiple layers of SU-8 to achieve the layered effect and the high aspect ratio of the pillar is prone to slant or twist. The fabrication process is difficult to maintain the quality.
U.S. Pat. No. 6,663,820 disclosed another method of using lithography and photolithography to fabricate polymer microneedle array, as shown in FIG. 4. This method has the advantages of rapid fabrication of micromold and microneedle, and low fabrication cost of the material and process. However, the flat-tip microneedles are still limited in the application. In addition, the polymer microneedles of this method do not have microchannels or reservoirs, and require additional fabrication process to attach the microchannels and reservoirs, if necessary. It is, therefore, difficult to have this method applied for mass production.
Numerous methods of fabricating microneedle array have been proposed. Regardless of the material used, the object of the microneedle array includes the capability to penetrate the human skin for micro-injection or micro-sampling painlessly, easy to fabricate, low in fabrication cost and safe to use.